The present invention relates to an equalization amplifier, and more particularly provides an equalization amplifier such as may conveniently be used for providing RIAA equalization of an output signal provided by a phonographic turntable.
Equalization amplifiers are circuits having nonlinear transfer characteristics designed to compensate for an undesired amplitude-frequency or phase-frequency characteristic of a system. Equalization amplifiers are used, for example, in phonographic record playing systems since the amplitude-frequency characteristic of the signal provided by the transducers in such systems (i.e., the phono cartridge) is not the same as the amplitude-frequency characteristic of the audio signal from which the phonographic recording was originally derived. This is because the audio signal is deliberately treble boosted during recording in order to improve signal-to-noise ratio during playback of the phonographic record. Furthermore, the bass frequencies of the played-back signal are severly attenuated due to inherent limitations in the phonographic recording/playback process.
Phono equalization amplifiers are designed to have nonlinear transfer characteristics in accordance with standards established by the Record Industry Association of America (RIAA) in order to eliminate this distortion on playback. The RIAA nonlinear transfer characteristic is essentially a low pass structure which begins to roll-off at 50 Hz, flattens out at 500 Hz, and begins to roll-off again at 2.1 KHz. The roll-off frequencies of 50 Hz and 2.1 kHz are known as "pole" frequencies, whereas the frequency at which the characteristic flattens out (i.e. 500 Hz) is known as a "zero" frequency.
In order to provide high quality reproduction of signals derived from phonographic records, it is of course desirable that the RIAA equalization amplifier provide as low a level of distortion as possible. Present RIAA amplifiers include but a single amplifier having frequency responsive components associated with it for tailoring the transfer characteristic of the amplifier to RIAA specifications. The gains of such RIAA equalization amplifiers are generally limited in order to prevent stability problems from arising. This limited amplifier gain also leads to inadequate bass response and excessive distortion, however. It would be desirable if an equalization amplifier could be provided which avoided these problems.